Conventionally, there has been known a visible light communication system, configured to transmit a communication signal with illumination light. In the visible light communication system, when plural lighting apparatuses are located in a predetermined space and each lighting apparatus has a visible light communication function, communication signals transmitted from the lighting apparatuses may interfere with each other, and accordingly, there is a case where a receiver cannot accurately receive communication information.
In view of this, there has been known an optical transmission system, in which a receiver can accurately receive communication information even in such a space where illumination light including a communication signal is output from each of plural lighting apparatuses (e.g., see JP 2005-176257 A). In this optical transmission system, a lighting apparatus is provided with a receiving unit that is configured to receive a communication signal included in illumination light output from another light source. The lighting apparatus is configured to determine whether or not the communication signal received by the receiving unit agrees with a communication signal to be output from its own light source, and output again an identical communication signal if those communication signals disagree with each other.
However, the optical transmission system in the above-mentioned document needs to provide the lighting apparatus with the receiving unit configured to receive the communication signal, and accordingly, the cost for the system configuration is increased. On the other hand, there is a method of synchronizing, with a single signal, plural lighting apparatuses in the same area to prevent interference of the communication signals. According to this method, one communication signal circuit is connected to a lighting circuit of each lighting apparatus through an electric wire, and the communication signal circuit is configured to output, to each lighting apparatus, a dimming signal on which a modulation signal for modulating light intensity of each light source is superposed. However, in this method, because a relatively-large current, on which the modulation signal is superposed, flows through the electric wire connecting the communication signal circuit and each lighting apparatus, radiation noise increases and accordingly there is a case where the communication signal circuit cannot accurately transmit the modulation signal. In addition, because general purpose electric wires and general purpose lighting apparatuses are mostly non-insulated type of things, it is not easy to secure insulation properties when connecting those.